I think you kind of answer your own question. Social media is just a gossip page on steroids. None of their features are worth paying for; at least, not enough people are willing to pay for it. They realized given showing ads and charging customers, showing ads probably generates 10x revenue for them. Plus, what's to stop another person to offer a free reddit clone?
People pay for junk food all the time. Maybe internet companies are being too greedy and they are pricing themselves incorrectly.
Would you pay $2 USD for all of reddit for a year?
How many users does reddit have? It'd probably be a lot of money, but not super profitable. Hence they use adds because they won't shift on price.
The alternative monetization model is usually subscriptions / freemium features. These don't usually work out for social media sites as people are not willing to pay for user generated content, and the main draw is the content itself so freemium features are usually not high-value enough. Thus, the only remaining viable model is ads.
Reddit's model is based on unpaid moderators managing most of the site. There's already controversy around a few moderators controlling all the large communities, karma farming and behind the scenes deals. Introducing money into the mix will just result in even more abuse as moderators try to take their own cut. It's a very messy eco-system that money will just make even messier.
If you think moderators won't take advantage of their position to get a cut of that money then you're living in fantasy land. Or just sell their accounts to others who will do that.